Exhaust gas recirculation has become increasingly significant in recent years, particularly for internal combustion engines. Its positive impact on exhaust gas behavior means that exhaust gas recirculation has an important key role with regard to meeting and complying with the stringent exhaust gas standards required now and in the near future. For internal combustion engines in particular, which are operated with a high proportion of air, which means that the proportion of nitrogen oxides (NOx) in the exhaust gas increases significantly, this technology allows a reduction of up to 80% of nitrogen oxides to be achieved.
To satisfy exhaust gas limit values, the mixture (fresh air/fuel ratio) in the combustion chamber must be set so that an optimal compromise between NOx and particle emissions results. To this end the control unit of the internal combustion engine changes the fresh air/fuel ratio, for example by changing the EGR rate or adjusting the electronic throttle control (ETC), so that the required exhaust gas composition is achieved.
To regulate a combustion engine, e.g. a charged diesel engine, to achieve optimal emissions in particular, it is extremely important to have precise knowledge of as many operating parameters of the engine system as possible. For a combustion engine with exhaust gas recirculation such an operating parameter is for example the exhaust gas recirculation mass flow, i.e. the mass flow of exhaust gas emitted by the combustion engine, which is fed by way of an exhaust gas recirculation line to a mixing point, where the exhaust gas is mixed with fresh air taken in, in order to feed the resulting fresh air/exhaust gas mixture to the combustion chambers of the combustion engine. The so-called exhaust gas recirculation rate, i.e. the quotient of the fresh air mass flow taken in and the exhaust gas recirculation mass flow, is also important for compliance with exhaust gas requirements.
Precise metering of the exact quantity of air and fuel has become increasingly important for lean-operation, low-emission combustion engines due to the high emission intensity. A common means of reducing the emissions of current engines (gas and diesel engines) here too is exhaust gas recirculation in particular, which reduces the proportion of oxygen in the intake air, thereby reducing nitrogen oxide emissions. It is disadvantageous here that as the oxygen content decreases, combustion tends to cause more soot to form. This results in the known “particle/nitrogen oxide tradeoff” with diesel engines, which refers to a mutual dependency of nitrogen oxides and particle emissions as a function of the exhaust gas recirculation rate. Precise setting of the recirculation rate during operation is therefore extraordinarily important for compliance with emission limits.
With current regulation models either the quantity of fresh air or the quantity of recirculated exhaust gas is generally regulated, with the result that the respective other mass flow is set according to the dependency:Fresh air mass+exhaust gas mass=absorption capacity of engine  (1)
In this process a given absorption capacity is generally set at the respective load point according to equation (1), in other words the following relationship results:Fresh air mass+exhaust gas mass=K1 (constant)  (2)
This relationship also applies in principle to charged engines and to throttled operation. The only difference compared with pure intake operation here is the fact that the absorption behavior can be set according to the known gas law:p·V=mRT  (3),wherep is pressure,V is volume,m is mass,R is the gas constant andT is temperature,in other words the mass or volume flow is a function of the intake pipe pressure. In principle equation (2) still applies, as the target intake pipe pressure also represents a fixed calibration variable and the target absorption capacity is therefore predetermined uniquely for each load point.
In practice this means that one of the two mass flows is regulated according to equation (2) by a precontrol target value, which is lower than the absorption capacity, and the remaining volume is filled by the respective other flow. This means that all the tolerances influencing the constant K1 have a negative impact on this filling quantity and therefore ultimately on the accuracy of the metered ratio of fresh air and recirculated exhaust gas. The essential parameters influencing K1 result from the production tolerances for the manufacture of the engine and air supply system, the change in operation, e.g. due to deposits in the intake system, in the case of charged engines due to the overall tolerance of the charging process (turbocharger, sensor system, regulator and/or precontroller, etc.) and in the case of throttling due to throttling accuracy.
In the case of current turbo-diesel engines the fresh air value and charging pressure are generally predetermined, with the latter frequently only being proposed and therefore being relatively inaccurate. The exhaust gas mass flow is set automatically and is determined from the difference between the target absorption capacity K1 and the current fresh air value as well as all the tolerances influencing K1. The negative impact of these tolerances on emission behavior is considerable here.